The most widely used technology at present for producing SiC whiskers is chemical vapor deposition (CVD). This process is often implemented using solid or gaseous starting reactants and employing overall chemical reactions such as: EQU SiO.sub.2 +3C .fwdarw.SiC+2CO or EQU SiCl.sub.4 +H.sub.2 +CH.sub.4 .fwdarw.SiC+H.sub.2 +4HCl.
As a general rule, to grow SiC whiskers, that is to say fine, elongated monocrystals, the process is carried out in the presence of elements that are called catalysts or solvents. As catalysts, use is often made of the transition metals Fe, Ni, Cr or Mn, but other elements such as Al, La, etc. can also be used. These elements can be introduced specially into the reaction zone or they may be present, as impurities, in the starting components or reactants. It is also possible to introduce the solvent elements concerned in the form of various compounds, for example oxides, if there exist conditions, in the whisker production process, that ensure that the compound in question is transformed into a free element.
The essential part played by the solvent or catalyst element is to form solutions, under SiC whisker production conditions, in the form of melted masses containing silicium and carbon, of the Me--Si--C type, with Me representing the metal or the metals playing the part-of the solvent or catalyst. In such a case, chemical vapor deposition takes place primarily on the surface of the droplets of melted mass according to the so-called VLS (Vapour-Liquid-Solid) process, which ensures the unidirectional axial growth of the SiC in the form of whiskers from the gaseous phase.
What is peculiar to all processes for the production of whiskers, including SiC whiskers, is that they have to be conducted at low supersaturation levels that are characteristic of one-dimensional growth. If, on the contrary, low supersaturation levels are not used, isometric equiaxial crystals, polycrystalline films or even powdery precipitates are formed instead of whiskers. Consequently, it is required for the process to be conducted with a gaseous phase containing a low concentration of reactants. The result of this requirement is an extremely low yield. For instance, in the case of SiC whiskers, the yield is generally from 0.1 to 1.0 g/m.sup.2 .h and, in exceptional cases, it reaches 3 to 5 g/m.sup.2 .h.
To overcome this drawback in industrial-scale production of SiC whiskers, they are grown within porous bodies forming substrates and having a high specific area.
In such cases, as a rule, the following reaction is used: EQU SiO.sub.2 +3C.fwdarw.SiC+2CO.
It is also possible, however, to "purge" the porous substrate with a gaseous mixture of the SiCl.sub.4 +H.sub.2 +CH.sub.4 type.
As a substrate material, use is made, in these two cases, of silica (SiO.sub.2) and of carbon disaggregated using different methods, or natural or synthetic substances containing silica and carbon. The solvent elements are either present at the outset in the starting components of the substrate, or they are specially introduced into the substrate.
The use of a highly porous substrate, as described above, makes it possible to achieve economically acceptable productivity with industrial plants, but the length of the SiC whiskers is limited by the dimensions of the pores in the substrate.
In general, the length of the SiC whiskers manufactured industrially amounts to a few tens of microns and not more than a hundred of microns.
There is known a process that enables the substrate to be disaggregated by introducing thereinto a polymer, carbon or ceramic fibers forming a skeleton, in order to form larger pores but, even then, the maximum length of the whiskers does not exceed 1 mm.
In view of the above, there is perceived to be a great need for SiC whiskers with lengths ranging from several millimeters to tens, or even hundreds, of millimeters, for different applications in the subsequent manufacture of continuous filaments, fabrics, mats, sheets and non-woven, semi-finished products.
To produce whiskers of such great length, it is necessary to provide free space having the requisite dimensions and, consequently, porous substrates become unacceptable. Use is then usually made of massive graphite and, less frequently, ceramic substrates. The whiskers are grown only at the geometrical substrate-gas interface. The chemical reactions used then are of the first or the second type indicated above. A solvent element is either deposited directly on the surface of the substrate, or placed separately in the vicinity thereof. With the processes that have just been described, it is possible to grow SiC whiskers having lengths of as much as 20 to 30 mm, or even more, but the productivity of the most efficient and widely used processes does not exceed 5 g/m.sup.2 .h. According to present evaluations, such processes can only become economically acceptable if productivity is increased by at least a factor of 2.
The need to use a massive substrate, that is to say one that is sufficiently thick in the case of graphite or ceramics, is an obstacle to the creation of large-scale industrial plants, and it will be appreciated that, in order to obtain sufficiently uniform products, it is necessary to ensure process parameters that are as constant as possible, in particular an equal concentration of reactants throughout the volume of the apparatus.
Separation of the whiskers from the substrate is the essential problem, as they are closely bonded thereto. The process described above is extremely difficult to mechanize and leads to contamination of the resulting product by the material of the substrate.